U.S. patent application Ser. No. 10/305,301 filed Nov. 26, 2002 now U.S. Pat. No. 6,887,599 discloses the use of an auxiliary load selectively attached to the electrical output of a fuel cell stack to limit individual cell voltage resulting from introduction of reactants during startup and resulting from consumption of reactants during shutdown of the fuel cell power plant, as described with respect to FIG. 1.
In FIG. 1, a prior art fuel cell system 100 includes a fuel cell 102 comprising an anode 104, a cathode 106, and an electrolyte layer 108 disposed between the anode and cathode. The anode includes an anode substrate 110 having an anode catalyst layer 112 disposed thereon on the side of the substrate facing the electrolyte layer 108. The cathode includes a cathode substrate 114, having a cathode catalyst layer 116 disposed thereon on the side of the substrate facing the electrolyte layer 108. The cell also includes an anode flow field plate 118 adjacent the anode substrate 110, and a cathode flow field plate 120 adjacent the cathode substrate 114.
The cathode flow field plate 120 has a plurality of channels 122 extending thereacross adjacent the cathode substrate forming a cathode flow field for carrying an oxidant, such as air, across the cathode from an inlet 124 to an outlet 126. The anode flow field plate 118 has a plurality of channels 128 extending thereacross adjacent the anode substrate forming an anode flow field for carrying a hydrogen containing fuel across the anode from an inlet 130 to an outlet 132. Some cells also include a cooler 131 adjacent the cathode flow field plate 120 for removing heat from the cell, such as by using a water pump 134 to circulate water through a loop 132 that passes through the coolers 131, a radiator 136 for rejecting the heat, and a flow control valve or orifice 138. In some fuel cell systems the anode and cathode flow field plates and the cooler plate, such as the plates 118, 122 and 131, or the like are porous and used to both carry gases to the cell anode and cathode and to transport water into or away from each cell. In those systems, the coolant loop pump, such as the pump 134, should remain on during the shut-down procedure to prevent reactant channels from becoming blocked by coolant.
Although only a single cell 120 is shown, in actuality a fuel cell system comprises a plurality of adjacent cells (i.e., a stack of cells) connected electrically in series.
The fuel cell system includes a source 140 of hydrogen containing fuel, under pressure, a source 142 of air, an air blower 144, a primary electricity using device called a primary load 146, an auxiliary load 148, an anode exhaust recycle loop 150, and a recycle loop blower 152. During normal fuel cell operation, when the cell is providing electricity to the primary load 146, a primary load switch 154 is closed (it is shown open in the drawing), and an auxiliary load switch 156 is open. The air blower 144, anode exhaust recycle blower 152 and coolant pump 134 are all on, and a valve 166 in a fuel feed conduit from the fuel source 140 into the anode recycle loop 150 downstream of the recycle blower 152 is open, as is the valve 170 in the recycle loop 150 and the anode exhaust vent valve 172 in an anode exhaust conduit 174. An air inlet feed valve 158 in the conduit 160 is open. An air feed valve 162 in a conduit 164 from the air source 142 to a point in the recycle loop upstream of the recycle blower 152 is closed.
Thus, during normal operation, air from the source 142 is continuously delivered into the cathode flow field inlet 124 via the conduit 160 and leaves the cell outlet 126 via a conduit 176. Fresh hydrogen containing fuel from the pressurized source 140 is continuously delivered into the anode flow field via the conduit 168, which directs the fuel into the recycle loop 150. A portion of the anode exhaust, containing depleted fuel leaves the anode flow field through the vent valve 172 via the conduit 174, while the recycle blower 152 recirculates the balance of the anode exhaust through the anode flow field via the recycle loop in a manner well known in the prior art. The recycle flow helps maintain a relatively uniform gas composition from the inlet 130 to the outlet 132 of the anode flow field, as well as returning some water vapor to the cell to prevent dry-out of the cell in the vicinity of the fuel inlet. The hydrogen in the fuel electrochemically reacts in a well-known manner during normal cell operation to produce protons (hydrogen ions) and electrons. The electrons flow from the anode 104 to the cathode 106 through an external circuit 178 to power the load 146, while the protons flow from the anode 104 to the cathode 106 through the electrolyte 108.
To avoid significant cell performance decay as a result of corrosion of the cell catalyst and catalyst support, the following procedure may be used to shut down the cell: The switch 154 is opened, disconnecting the primary load from the external circuit. The valve 166 remains open permitting the flow of fuel to the anode flow field. The air inlet feed valve 158 is preferably closed, as well as the anode vent valve. The recycle flow valve 170 may remain open and the recycle blower 152 may remain on in order to continue to recirculate the anode exhaust through the cell. This prevents localized fuel starvation on the anode. The switch 156 is then closed, thereby connecting the small auxiliary resistive load 148 across the cell in the external circuit 177, 178. With the switch 156 closed, the usual cell electrochemical reactions continue to occur such that the oxygen concentration in the cathode flow field is reduced.
The valve 158 may be partially open during the period of auxiliary load application to prevent the pressure in the cathode chamber from dropping below ambient pressure.
The auxiliary load 148 is preferably sized to lower the cell voltage from its open circuit voltage of about 0.90–1.0 volts to about 0.20 volts in about 15 seconds to one minute. The size of the load necessary to accomplish this will depend upon the particulars of the cell design, such as number of cells, size of cells, and the maximum volume of hydrogen and air within the anode flow field and any fuel manifolds or the like and also within the cathode flow field and any air manifolds or the like, respectively. Because of the low level current production resulting from application of the auxiliary load prior to the commencement of the fuel purge, the magnitude of the “reverse currents” believed to cause cell performance decay during shut-down will be lower during the air purge step.
Once the cell voltage has been reduced by a predetermined amount (preferably to a cell voltage of 0.2 volts or less), the switch 156 may be opened, or it may remain closed, and the valve 166 is closed during all or part of the remainder of the shutdown procedure. The recycle valve 170 is closed and the recycle blower turned off to prevent further recirculation of the anode exhaust. The anode exhaust vent valve is opened, and the air flow valve 162 is then opened to allow air from the source 142 into the recycle loop immediately downstream of the valve 170 and just upstream of the recycle blower 152.
Assuming that the cell has been shut down in accordance with the foregoing procedure, there would only be dilute hydrogen and nitrogen within the anode and cathode flow fields. To restart the fuel cell system 100, the auxiliary load switch 156 is closed (if open) to connect the auxiliary load 148 in the external circuit 177, 178, the coolant loop valve 138, if closed, is opened and the pump 134 is turned on. The air flow valve 158 is closed; and the blower 144 is off. The anode exhaust vent valve 172 is open and the air flow valve in the conduit 162 is closed. The recycle flow valve 170 is also open, and the recycle blower 152 is on. The fuel flow valve 166 is opened to allow a flow of pressurized hydrogen from the source 140 into the anode flow field. The auxiliary load switch 156 is opened once hydrogen reaches the anode exhaust 172. The air flow valve 158 is opened, the air blower 144 is turned on, and the switch 154 is closed to connect the primary load across the cell 102. The cell may now be operated normally.
There are several problems with the prior art fixed auxiliary load voltage limiting function described hereinbefore. During startup, the fixed auxiliary load is connected to the fuel cell stack when the cell stack voltage rises above a predetermined level, and then is disconnected after a predetermined length of time, or, in some embodiments, after the DC current decays below a specified level. The load remains constant throughout that time.
If the same auxiliary load is used both for startup and shutdown, then either startup, shutdown or both will not be optimized as a result. During the shutdown process, a separate auxiliary load, with a different resistance and power rating may be connected to the fuel cell until the cell stack voltage decays below a specified level, after which it is disconnected. The use of two separate and distinct auxiliary loads for voltage limiting during startup and shutdown are costly to implement and package.
During startup, the anode fuel is introduced into the entire fuel cell stack in a fashion where it is not of a uniform concentration within each of the fuel cells, nor has is progressed the same amount through the fuel flow fields in each of the fuel cells. If current is drawn through a cell which does not have adequate fuel, the result is called “fuel starvation” which creates the negative voltages in an individual cell with inadequate fuel. Therefore, the open circuit cell stack voltage used to engage the auxiliary load must be set high enough to ensure that all cells have sufficient fuel. This results in many individual cells rising to a voltage level greater than that at which undesirable fuel cell decay mechanisms occur.
The prior art system is not sufficiently robust to accommodate variations between individual powerplants caused by manufacturing tolerances, or variations in powerplants that occur throughout the power plant operating lifetime. The control parameters used to connect and disconnect the auxiliary load must be conservative in order to avoid negative individual cell voltages. This means that many individual cells rise to a voltage level greater than desired to minimize cell decay mechanisms.
Auxiliary loads used for voltage limitation during startup and shutdown must be sized to dissipate the full energy generated by the fuel cell during these transition periods. Design must account for worst case environmental conditions, such as the temperature of the coolant and the temperature of the ambient environment, which results in costly over design compared with design adequate for normal, typical power plant operating conditions.
In utilizing either single auxiliary loads, or separate fixed auxiliary loads for startup and shutdown, mitigation of the over-voltage conditions and resulting decay in fuel cell performance has been sufficiently unsatisfactory so as to direct attention to formulation of fuel inlet manifolding systems, to provide a more uniform introduction of fuel to all the cells of a fuel cell stack. These systems are costly and too cumbersome for use in compact applications, such as in electric vehicles powered by a fuel cell power plant.